It is proposed to continue a research program designed to provide information about DNA base damage and its effects. The quantities and types of damage to thymine and adenine bases produced by ionizing radiation in vivo will be determined in bacterial strains selected for their known radiation sensitivities and for their genetically characterized ability to repair radiation damage. Specific products of base damage, such as thymine hydroperoxides, will be measured as a function of radiation dose, and time after irradiation during the period in which radiation damage is thought to be repaired. Since it has been found that undamaged base residues (bases, nucleosides, oligonucleotides) are released from DNA immediately after irradiation, these residues will be quantitated as a function of bacterial strain and postirradiation incubation time. The effect of 02 and several cis platinum anti-tumor drugs on the quantities and types of DNA base damage as well as on base residue release will be determined. Radiation effects on carbon-13 enriched DNA in vivo and in vitro will be studied by NMR techniques. BIBLIOGRAPHIC REFERENCE: R.C. Richmond and J.D. Zimbrick, "Radiation-induced Base Residue Release from Bacterial Cell DNA," Abstract of paper to be presented at annual meeting of the Radiation Research Society, San Francisco, Ca., June 27-July 2, 1976. Teymoor Gedayloo, John D. Zimbrick and Wesley P. Unruh, "Hindered Rotation of Methyl Protons in 5-thymyl Radicals in Acidic Glass at 1.2 degrees K," abstract of paper to be presented at annual meeting of Radiation Research Society, San Francisco, Ca., June 27-July 2, 1976.